The present invention is directed to a polymeric composition to be used in producing foam. Specifically, the polymeric composition is comprised of a recycled or offgrade conventional linear polypropylene, a high melt strength polypropylene, and a rheology modifier resin.
Low density foam, such as polystyrene foam, which has a density in the range of 10 to 160 kg/m3, is commonly made by combining a physical blowing agent with a molten polymeric mixture under pressure and, after thorough mixing, extruding the combination through an appropriate die into a lower pressure atmosphere.
There are two polymeric foams that are commonly produced. The first foam is made firm polystyrene and the second foam is made from low density polyethylenes (LDPEs). Pure polystyrene foam is fragile and is easily deformed permanently by application of compressive or shear force and thus can not be used in many applications that require structural strength. Polystyrene foam has a maximum service temperature just below the boiling point of water and is readily attacked by many common oils found in foodstuffs, and although commonly used in foodservice packaging, can not be used as packaging for many hot foods.
Polymeric foam composed of LDPE are generally considered to be resilient and non-brittle, which are desirable properties. The LDPE foams, however, have disadvantages such as a maximum service temperature below the boiling point of water and a melting point at just above the boiling point of water, which render it unsuitable for many cushioning applications like transportation vehicle components that would otherwise benefit from its resiliency. LDPE foams also have a low flexural strength and despite their excellent chemical resistance to common food oils, are thus not suitable for making packaging for food containers.
Recently, some propylene polymer foams with density below 160 kg/m3 have been reported which have properties that overcome some of the cited shortcomings of both polystyrene and LDPE foams. Because of the low melt strength of conventional linear polypropylenes, which basically have a linear molecular structure, such low density foams produced from propylene polymer require that the major fraction (51 to 100%) of the polymeric composition be a high melt strength polypropylene (HMS-PP). However, in order to achieve the lowest density foams, most processes require 100% HMS-PP for the polymeric composition Since the manufacturing process for HMS-PP polymer itself is complex and costly, HMS-PP is a specialty polymer. HMS-PP is thus sold at a premium typically 60-100% above virgin conventional polypropylenes, which are commodities. Therefore, the cost of these propylene polymer foams has limited its competitive use in applications such as food packaging; transportation components such as automotive trunk liners; construction materials such as pipe wrap and flooring underlayment; sports and leisure equipment such as bodyboards, life vests, seat cushions, and floatation devices; and protective packaging of delicate and fragile high surface quality goods. Furthermore, the current process requirements for propylene polymer foams severely restrict the incorporation of recycled polypropylene from post-consumer and post-industrial sources. Incorporation of recycled polypropylene into a polypropylene foam structure would, of course, be very desirable as new regulations are introduced to reduce waste and to recover and reuse industrial materials.
Accordingly, a need exists for foams that overcome the above-noted shortcomings associated with existing foams.